Computer-aided drafting is a widely-used method of drawing devices and expressing concepts. Drafters generally create and edit drawings through the use of a drafting application that resides in a computer. A conventional drawing generated by using a drafting application is an assembly of discrete surfaces. For example, an isometric view of a cube may be illustrated by joining together three polygons at the appropriate edges.
The characteristics of a particular surface, such as its shape and orientation, are defined by curves. A curve is a graph of a mathematical function. Because a surface is defined by the curves, it is modified by adjusting, adding, or deleting the curves. Furthermore, the number of curves that define a surface identifies the method by which the surface is generated. For example, a surface that is generated by two non-intersecting curves that are joined at their ends by a third curve to define an enclosed space is referred to as being generated using a 2×1 method or a 1×2 method, depending on the reference point of the viewer.
In working with a surface, drawing packages impose modification restrictions on each surface based on the method that was used to generate the surface. Drawing packages also impose modification restrictions on each curve based on the mathematical characteristics of the equation that defines the curve. Because of these restrictions, a user must keep track of the method that was used to generate each surface that the user wants to modify. Also, the user must keep track of the mathematical characteristics of each curve that the user wants to modify. These requirements make it inconvenient and inefficient for the user to modify or edit a drawing.